A technique in which electric power is supplied to a load device by a non-contact system is known. As a product into which the technique is applied, a mobile phone charging system has become popular in general. Furthermore, in recent years, the non-contact power supply system is practically used even as a power supply system for electric vehicles, and various standards are established in view of practical operations.
There are various types of non-contact power supply system. A power supply system for electric vehicles is a resonance-type non-contact power supply system, which is shown in FIG. 1, which greatly attracts attentions and whose basic principle is developed and demonstrated by MIT (Massachusetts Institute of Technology) (for example, refer to JP-A-2009-501510). The resonance-type non-contact power supply system shown in the figure includes a resonance system of a high frequency power supply, resonance coils (primary and secondary resonance coils) and a load that transmits electric power non-contactly. Specifically, power-transmission-side (primary side) devices include a high frequency power supply, a primary coil, and a primary resonance coil. Power-receiving-side (secondary device) devices include a secondary resonance coil, a secondary coil and a load. The power-transmission-side devices and the power-receiving-side devices in the system have an advantage of being able to supply electric power to a place spaced several meters with a high transmission efficiency (sometimes around 50%) by being magnetically coupled (electromagnetically coupled)) by resonance.
In the technique of MIT shown in FIG. 1, the resonance system is assumed to be configured with “a power supply part (the high frequency power supply and the primary coil), a resonance part (the primary resonance coil and the secondary resonance coil), and a load part” (the secondary coil and the load). However, additional components become necessary when the non-contact power supply system is mounted in an electronic device or an automobile power supply system. A system configuration example where the system of FIG. 1 is mounted in a real system is shown in FIG. 2. As shown in the figure, in the real system, a transmission path between the power supply and a primary resonance coil part and a transmission path between a secondary resonance coil part and the load are necessary. When parallel lines are assumed as the transmission path, because variation of the characteristic impedance is large and transmission loss is large, the non-contact power supply system is applicable substantially only to a small-scale system.
For example, in the electromagnetic induction method, the above problem is partly solved by a technique using Litz wires (refer to JP-A-2010-40699 and JP-A-5-344602).
According to the technique using Litz wires disclosed in JP-A-2010-40699 and JP-A-5-344602, there is a problem that the variation of the characteristic impedance cannot be reduced although the transmission loss can be reduced. Therefore, in the frequency band (several MHz to several 10 MHz) used in the resonance method, there are few improvement effects. As a result, a coaxial cable for which the variation of the characteristic impedance is small, and the transmission efficiency is high is often used for the transmission path (transmission line) of a large-scale system such as the system used for charging vehicles. However, as shown in FIG. 3, when coaxial cables are used in a system in which the resonance method is used, there are the following problems.
(1) When a coaxial cable is used for the transmission path, an electric current flows through not only the inner side but also the outer side of a coaxial cable outer conductor 64 of the primary coaxial cable (the power-transmission-side coaxial cable 60), and a radiated electromagnetic field occurs.(2) Because part of the electromagnetic field from a primary coil 30 is coupled with the coaxial cable outer conductor 64 and an induced current flows, a radiated electromagnetic field occurs.(3) Because all of the electromagnetic field from a secondary resonance coil 45 is not necessarily coupled with a secondary coil 40, part of the electromagnetic field is coupled with a coaxial cable outer conductor 74 of a power-receiving-side coaxial cable 70, and an induced current flows, a radiated electromagnetic field occurs.
When these problems are addressed from the viewpoint of transmission efficiency, because of the idealized model in FIG. 1, it is assumed that all the electromagnetic energy is supplied to the load, but part of the electromagnetic energy is lost in the constructions of FIGS. 2 and 3.
A technique is considered to cover the whole system with shields 99 as a general measure, as shown in FIG. 4. However, the induced current that is the cause of the transmission loss cannot be prevented from occurring by this technique. Further, there are the following problems: power supply operations are disturbed, the weight of the overall system increases, and the system is difficult to be mounted. Because particularly when the system is mounted to a vehicle or the like, the energy consumption efficiency drops remarkably with the weight increase of the vehicle, another more practical technique is demanded.